1. Field of Invention
This invention is concerned with adjustable value tuning capacitors for microwave circuits, and more particularly to circuits which can easily be interconnected to achieve any desired one of a plurality of capacitance values.
2. Description of the Prior Art
One of the most effective tuning techniques for a class of miniature microwave circuits that are constructed by means of lumped elements is to vary the bonding configuration of isolated capacitors. The desired effect is achieved either or both of selecting the location of the bonds, which effectively varies the electrical length of the circuit, or by selecting the number of bonds, which effectively varies the total capacitance connected into the circuit. It is common to have an array of differently valued capacitors fabricated near the facing edges of the two circuit conductor electrodes to be capacitively joined. The capacitors are fabricated with dielectric pieces positioned atop one or both electrodes and electrodes positioned on each dielectric piece. Where the capacitors are only on one electrode, an interconnecting wire bond is positioned between a selected capacitor and the opposite circuit conductor electrode. Where the capacitors are on both electrodes, an interconnecting wire bond is connected between a selected capacitor on one circuit conductor electrode and a selected capacitor on the opposite circuit conductor electrode. Tuning achieved by bonding multiple capacitors, while theoretically possible, is not practical because of the excess parasitics produced by the wire bonds.
The main problems with the prior art are: (a) the bonding over the top electrodes stresses the dielectric film often causing the capacitor to fail; (b) the tuning is very time consuming since each change in tuning configuration requires the removal of the circuit from the measurement setup and often also from holding fixtures and its placement into a bonding machine; and (c) bond wires introduce undesired electrical characteristics which are not well controlled and vary from circuit to circuit